Structural Attributes of Two Old-Growth Cross Timbers Stands in Western Arkansas

Comprised of largely non-commercial, xeric, oak-dominated forests, the Cross Timbers in Arkansas have been heavily altered over the last two centuries, and thus only scattered parcels of old-growth timber remain. We inventoried and mapped two such stands on Fort Chaffee Military Training Center in Sebastian County, Arkansas. The west-facing Christmas Knob site is located on an isolated hill, while the southerly-facing Big Creek Narrows site is on a long, narrow rocky outcrop called Devil's Backbone Ridge. These sites occupied rocky, south- to southwest-facing sandstone-dominated slopes, with primarily post oak (Quercus stellata) and blackjack oak (Q. marilandica) overstories. Post oak dominated the largest size classes at both sites. Increment cores indicated that some post oaks exceeded 200 y of age, and tree-ring dating also confirmed an uneven-aged structure to these stands. Both locations had irregular reverse-J shaped diameter distributions, with gaps, deficiencies, and excesses in larger size classes that often typify old-growth stands. On average, the post oaks at the Big Creek Narrows site were taller, larger in girth, and younger than those on the Christmas Knob site, suggestive of a better quality site at Big Creek. The application of neighborhood density functions on stem maps of both sites found random patterns in tree locations. These stands are very similar in their structure to old-growth examples in other parts of the Cross Timbers ecoregion.

INTRODUCTION

The Cross Timbers ecoregion occupies southeastern Kansas, eastern and central Oklahoma, and north-central Texas and has been mapped as far east as extreme westcentral Arkansas (Bruner, 1931; Rice and Penfound, 1959; Kuchler, 1964; Hoagland et al, 1999). This ecoregion is considered transitional between the hardwood and pine-dominated forests of eastern North America and the prairies of the Great Plains to the west. The Cross Timbers are mostìy post oak ( (hiercus stellata) and blackjack oak ( Q. marilandica) , with few other tree species contributing to total stand density. Because of fheir ecotonal nature, the Cross Timbers often reflect parts of adjacent ecoregions - some patches may be more mesic and therefore dominated by denser forests, while other stands include mixtures of grassdominated woodland, savanna, and pockets of prairie.

Too poor in log quality for timber production and unsuitable for conventional row-crop agriculture, large areas of old-growth Cross Timbers can still be found, although only a small fraction is in some protected status (Stahle and Hehr, 1984; Therrell and Stahle, 1998). Oldgrowth remnants have been studied in the center of the range of this ecoregion (e.g., Johnson and Risser, 1975; Therrell and Stahle, 1998; Peppers, 2004; Clark et al, 2005; Burton et al, 2010). Considerably less information is available for old-growth forests along the fringes of the ecoregion, especially on the margins of the steep east-to-west precipitation gradient that helps to define the Cross Timbers. Due to relatively high precipitation, the easternmost Cross Timbers sites are usually more limited in their distribution across the landscape than those in the drier portions of the ecoregion. These xeric oak stands are found primarily along narrow sandstone-controlled ridges and prominences, especially on thin, rocky soils with south and south-westerly aspects.

Kuchler (1964) mapped the Cross Timbers ecoregion as extending into Arkansas, although not all later maps have included the state (e.g., US EPA, 2011). Across this ecoregion, agriculture (primarily livestock grazing), oil and gas development, and residential growth have reduced what was once many thousands of contiguous hectares of suitable habitat for old-growth Cross Timbers. Describing the structural attributes of intact old-growth Cross Timbers stands before they are lost to development or altered by climate change or invasive species is important if they are to be protected or restored. …

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